1. Field of the Invention
The present invention relates to an image display system which displays on a screen of a television receiver with a large aspect ratio a picture smaller in size than the TV screen.
2. Prior Art
As an increasing number of television sets incorporate a large screen, flickers and blurs that so far have not been noticeable because of visual characteristic of small screens show up conspicuously. There are growing demands for improving the picture quality of the television sets and, to meet these demands, a variety of kinds of high picture quality television system are being developed.
An improved-definition TV or IDTV, for example, is compatible with the current NTSC (National Television System Committee) standard and designed to improve picture quality by introducing a sophisticated signal processing in the receiver side. Among the major features of the IDTV are an elimination of cross interference between color and luminance signals by a motionadapted separation of brightness and color signals and a progressive scanning of an increased number of noninterlaced lines. An extended-definition TV or EDTV, like the IDTV system, removes factors responsible for image quality degradation experienced with the standard NTSC system and introduces improvements on the image sending side to enhance the picture quality and resolution. In addition to the motion-adapted separation of brightness and color signals and the progressive scanning, the EDTV has also realized improvements on the horizontal resolution by expanding the bandwidths of brightness and color signals, elimination of ghost by inserting the primary color signals and introduction of a wide aspect ratio.
While both of the two systems comply with the current NTSC system, a high-definition TV or HDTV aims at the next generation ideal television system, not restricted by the conventional NTSC standard. Not only does the HDTV system have far better image and sound qualities than the present NTSC system but it also provides highly visual and psychological effects which give television viewers an impression as if they were really present in the scene.
In providing viewers with the vivid feeling of being present in the scene, a horizontally wide screen is desirable. The EDTV and HDTV systems therefore employ a screen of a 16-to-9 aspect ratio, considering the results of studies on visual and psychological effects of the screen. When a picture image of the NTSC system with a 4-to-3 aspect ratio is displayed on the screen of the EDTV receiver with a large aspect ratio without changing the aspect ratio of the former, or when it is up-converted and displayed on the HDTV receiver, or when a picture of the HDTV system is down-converted to produce a picture of 4-to-3 aspect ratio of NTSC system and displayed on the NTSC or EDTV receiver with a large aspect ratio, blank areas are formed on the right and left of the screen, as shown in FIG. 3. The blank areas on the screen appear at black level, so that if this kind of display with unmatched aspect ratio continues for a long period of time, phosphor materials on the image area and on the blank area deteriorate to different fatigue levels. When in this condition the whole area of the screen is used, a portion of the screen corresponding to the blank area appears brighter than the remaining portion corresponding to the image area, forming a line at the boundary between the two areas. This is the result of an uneven degradation of CRT.